In soldering electronic components, circuits, equipment, etc., various kinds of fluxes are used together with soldering material so as to improve the efficiency of the soldering operation, to secure the soldered connections and to improve the long-term reliability of the connections. Conventionally, three kinds of fluxes are mainly used: (1) water soluble flux which is made from inorganic acids, organic acids, amine hydrohalides, etc., (2) natural rosin and (3) activated rosin having a halogenated compound, e.g., an amine hydrohalide, incorporated with natural rosin.
Various additives have been added to rosin to form an activated rosin flux. Keto acids, for example, levulinic acid, have been combined with rosin (see U.S. Pat. No. 2,361,867). Adipic acid has also been combined with rosin in the formation of a flux core. Also, it has heretofore been known to combine adipic acid and levulinic acid with rosin to form a solder flux.
Water-soluble flux is very useful in removing oxides from the metal surface to be soldered. However, there are disadvantages in that the water-soluble flux is apt to destroy metallic material and to leave residues which corrode the soldered parts after soldering. The corrosion impairs the reliability of the soldered parts with respect to electrical and mechanical properties. This undesirable property of water-soluble flux can only be solved by using an extremely efficient flux residue removal and cleaning process.
Natural rosin which is available under a name of WW rosin (water white rosin) presents no problems with respect to corrosiveness, but is inferior as a soldering adjuvant.
Activated rosin has a stability similar to natural rosin and causes little corrosion at room temperature. Also, fully activated or liquid rosin activated (LRA) fluxes have a strong fluxing action at the soldering temperature due to activators, such as an amine hydrochloride, which are typically present in high concentrations such as 1 to 10 weight percent of the resultant flux. However, the fully activated rosin or LRA flux has disadvantages in that a corrosive gas is produced at soldering temperature and can harm the surface of metals such as copper, brass, etc. Moreover, the residues of the activated rosin combine with moisture and produce an acid which causes spontaneous corrosion similar to that produced by the water-soluble flux. Presently available fluxes containing organic hydrohalides, in the form of neutral salts, e.g., glutamic acid hydrochloride, either form corrosive metal halides at elevated temperature or the residues thereof combine with moisture at room temperature to form a corrosive acid and thus are used with possible deleterious effect for electrical soldering applications.
U.S. Pat. No. 2,898,255 reveals an activated rosin comprising a monocarboxylic acid, such as formic acid, combined with a dicarboxylic acid, such as glutamic acid. Such a flux, however, is too acidic and corrosive for practical use in electronics soldering operations. Such a flux is typical of an LRA flux which is corrosive, as evidenced by the standard copper mirror test outlined in the Electronics Industries Association Standard Number RS-402, for liquid rosin fluxes (approved Mar. 27, 1973).
A soldering flux which is superior in fluxing action and is free from corrosive action at room temperature as well as free from harmful residues is therefore desired. Such a flux is described in U.S. Pat. No. 4,168,996, issued Sept. 25, 1979. While the fluxes described in that patent meet the objectives as set forth above providing very high soldering efficiency, a specific problem of solder adherence to the dielectric has been encountered when this or other fluxes are used on specific types of substrates. The problem is most often observed when the substrate surface is an incompletely cured epoxy or rubber-modified epoxy. This solder sticking can result in electrical shorts leaving the circuits frequently beyond repair. I have now discovered a means of modifying the previously described solder formulation of U.S. Pat. No. 4,168,996 which is incorporated herein by reference to essentially eliminate solder sticking while still retaining the beneficial characteristics of the prior formulation.